In thermal imaging or printing, images are formed by heating heat-activatable materials in an imagewise manner. Such heating is commonly conducted by means of a thermal printhead, which consists of an array of electrically heatable elements, each of which is preferably activated by a computer in a time sequence designed to produce imagewise heating. The most common forms of thermal imaging are direct thermal imaging and thermal transfer imaging.
In thermal transfer imaging processes, an image is formed on a thermographic sheet known as a receptor sheet by selectively transferring an image forming material to the receptor sheet from another thermographic sheet, known as the donor sheet, using a thermal printhead. The three broad classes of thermal transfer imaging processes are described in U.S. Pat. No. 4,853,365 (Jongewaard et al). Typically, the donor sheet has a dye-donor layer disposed upon a thin, flexible substrate such as paper or polymeric film. Depending upon the type of thermal transfer imaging process desired, the dye-donor layer may take one of several forms, such as a meltable colored wax, a diffusing dye, or heat-activatable reactants which, when combined with other reactants incorporated into the receptor sheet, form a colored compound.
Generally, in direct thermal imaging processes a thermographic sheet having a dye-containing layer containing colorless forms of heat-activatable dyes and polymeric binder is heated in an imagewise manner by a thermal printhead. Upon application of heat, the colorless forms of the dyes are converted to their colored forms so that an image is formed in the dye-containing layer. Preferably, the thermal printhead directly contacts the dye-containing layer, however, many of the dye-containing layers contain compounds, such as the polymeric binder compounds, which soften or melt and stick to the printhead reducing printhead life and image quality.
Various materials have been described as being useful as substrates for thermographic sheets. For example, white-filled or transparent films of polyester (e.g., polyethylene terephthalate (PET)), polyethylene naphthalate, polysulphone, polystyrene, polycarbonate, polyimide, polyamide, cellulose ester (e.g., cellulose acetate and cellulose butyrate), polyvinyl chloride and paper have been described as useful. However, all of these materials have one or more disadvantages which reduce their suitability as substrates for thermographic sheets. For example, some of the materials have glass transition temperatures (T.sub.g) or melting temperatures (T.sub.m) which are lower than the temperature to which substrates would be heated during thermal imaging processes, resulting in image distortion. Some of the materials have poor optical properties (e.g., high coefficient of birefringence, high % haze or are inherently colored) rendering them unsuitable as substrates for certain thermal imaging applications, some have poor film properties (e.g., poor tensile strength and elongation at break), some have poor chemical resistance and some are hygroscopic.
Polyethylene terephthalate (PET) film has been preferred for use as substrates in thermographic sheets because it is a relatively low cost material, it is available in various thicknesses and it provides relatively good optical clarity and tensile strength. However, donor sheets made using PET film substrates have a tendency to soften or melt and stick on the thermal printhead, and friction between the PET film and the printhead can result in reduced printhead life and poor image quality. Generally, PET film of 4.5 to 6 micron thickness is used in donor sheets, but PET film of such thickness tends to dimensionally distort from the heat imparted by the printhead. Thus, donor sheets made using PET film cannot easily be reused or recycled, for example in thermal dye transfer processes. Since PET films of less than 4.5 micron thickness tend to wrinkle and tear during the thermographic element manufacturing process, e.g., on coating lines, very thin films (i.e., less than 4.5 microns) are not particularly practical for use in thermographic sheets.
One means of preventing sticking of the donor sheet to the printhead has been to select substrate materials which have softening temperatures higher than those encountered by the donor sheet in the printing process. For example, Japanese Patent application No. J6 1246-095-A, describes the use of copolymers containing acrylonitrile. However, none of the proposed materials have displaced PET film as the commercially preferred polymeric material for donor sheets.
In order to reduce sticking of thermographic sheets to the thermal printhead, antistick layers have been applied to the surface of the donor sheet contacting the thermal printhead and to the surface of the heat-activatable dye layer on thermographic sheets used in direct thermal imaging processes. For example, low surface energy compounds, such as fluoropolymers, silicones, waxes, fatty acids, and metal stearates, have been described as antistick coatings. Antistick compositions containing a low surface energy compound and a polymeric binder having a sufficiently high T.sub.g so the binder does not soften during the thermal imaging process have also been described.
One problem associated with the use of known antistick compounds or compositions is that many of the antistick compounds or compositions are not readily soluble or dispersible in commonly used organic solvents rendering such compounds or compositions difficult to use. Although some antistick compounds may be soluble in organic solvents and at the same time may exhibit antistick behavior (e.g., polymeric silicones), they may be very migratory, i.e., they spontaneously spread along surfaces for long distances, thereby contaminating large areas of the coating facilities, as well as the image-forming material and thermal imaging equipment. Further, when donor sheets are stored in roll form, some silicones may migrate from the side of the sheet to which they have been applied to its opposite side, where they may interfere with the thermal imaging process. Crosslinking or high degrees of polymerization of silicone polymers may be helpful in reducing migration, but because even small amounts of uncrosslinked silicones can have a significant negative effect upon imaging, it is difficult to achieve sufficient crosslinking. Waxes may easily be applied to the thermographic sheet, but they generally contaminate printheads to an unacceptable degree. One additional disadvantage of using an antistick layer in a thermographic sheet is that the application of such a layer requires an additional coating step.
Thus, it would be desirable to use a material as a donor sheet substrate which does not stick to the thermal printhead or dimensionally distort upon heating. However, such a material should also perform about as well as PET film in donor sheets. It would also be desirable to use, as an antistick layer, a composition possessing antistick properties which does not have the disadvantages of some of the compositions described in the art.